1. Field of the Invention
The present invention relates to a rubber composition, and more particularly, relates to a super rigid rubber composition comprising rubber, novolak-type phenolic resin and carbon black, and adapted to be used as a bead filler rubber of a tire.
2. Description of the Prior Art
There have been variously investigated the structure of bead portion of radial tire in order to satisfy the rigidity and durability required to tire. For example, it has been attempted to improve the dynamic performance and durability of tire by arranging a bead-reinforcing layer in the bead portion. However, this method has drawbacks that a large number of production steps are necessary and the productivity of the tire is very poor.
Japanese Utility Model Application No. 16,084/72, French Patent No. 1,260,138 and U.S. Pat. No. 4,067,373 disclose methods for improving the running performance and other property by arranging super rigid rubber in the bead portion. However, these methods do not substantially think of rubber, which can develop fully a function as a bead filler rubber subjected to complicated forces during the running of a tire and can give a sufficiently high durability to a rubber tire.
While, it is well known to use novolak-type phenolic resin in the production of rigid rubber compositions. However, these rubber compositions substantially concern nitrile rubber and neoprene rubber having a high compatibility with the resin. The nitrile rubber series of neoprene rubber series rigid rubber is very difficult to be vulcanized together with natural rubber, polybutadiene rubber and the like, which are commonly used as a rubber for tire, and therefore when the nitrile rubber series or neoprene rubber series rigid rubber is used as a tire part, the rubber is apt to be separated from natural rubber, polybutadiene rubber or the like, and can not be practically used.
In order to solve the above described drawbacks, the inventors have variously investigated how to produce a super rigid rubber composition by compounding novolak-type phenolic resin to natural rubber, polybutadiene rubber or the like, and found out the following facts. Novolak-type phenolic resin is essentially incompatible with natural rubber and other rubbers. Therefore, novolak-type phenolic resin is formed into spherical agglomerates during the kneading commonly carried out in the production of rubber, and has the filling effect only. This phenomenon still occurs even when various resins are used as a resin in place of novolak-type phenolic resin for producing a homogenous mixture of the resin with the rubber. However, when novolak-type phenolic resin is compounded to natural rubber or other rubber together with carbon black, the mixture of the resin and carbon black exhibits a reinforcing effect on the rubber, which is completely different from the reinforcing effect of the resin alone on the rubber or the reinforcing effect of the carbon black alone on the rubber, depending upon the mixing ratio of the resin to the carbon black, and a super rigid rubber composition having a durability remarkably superior to that of conventional resin-reinforced rubber can be obtained. That is, when natural rubber or diene series rubber is mixed with novolak-type phenolic resin, which is essentially incompatible with the rubber, the resin is separated from the rubber to form island-like large agglomerates in the mixture, but when a mixture of novolak-type phenolic resin and carbon black is compounded to natural rubber or other diene series rubber, the resin disperses uniformly in the rubber without forming large agglomerates similarly to the dispersed state in an ordinary mixture of rubber and carbon black. This action of carbon black to the resin is highly influenced by the mixing ratio of the carbon black to the resin and by the kind of the carbon black. Based on the discovery, the inventors have accomplished the present invention.